Method and system for high resolution digitized display

ABSTRACT

A method and system for increasing dynamic digitized wavefront resolution, i.e., the density of output beamlets, can include receiving a single collimated source light beam and producing multiple output beamlets spatially offset when out-coupled from a waveguide. The multiple output beamlets can be obtained by offsetting and replicating a collimated source light beam. Alternatively, the multiple output beamlets can be obtained by using a collimated incoming source light beam having multiple input beams with different wavelengths in the vicinity of the nominal wavelength of a particular color. The collimated incoming source light beam can be in-coupled into the eyepiece designed for the nominal wavelength. The input beams with multiple wavelengths take different paths when they undergo total internal reflection in the waveguide, which produces multiple output beamlets.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/826,315, filed on Nov. 29, 2017, U.S. Pat. No. 10,678,055, issued onJun. 9, 2020, entitled “METHOD AND SYSTEM FOR HIGH RESOLUTION DIGITIZEDDISPLAY,” which is a non-provisional of and claims the benefit of andpriority to U.S. Provisional Patent Application No. 62/428,510, filed onNov. 30, 2016, entitled “METHOD AND SYSTEM FOR HIGH RESOLUTION DIGITIZEDDISPLAY,” which are hereby incorporated by reference in their entiretyfor all purposes.

BACKGROUND OF THE INVENTION

Modern computing and display technologies have facilitated thedevelopment of systems for so-called “virtual reality” or “augmentedreality” experiences, wherein digitally produced images or portionsthereof are presented in a wearable device to a user in a manner whereinthey seem to be, or may be perceived as, real. A virtual reality, or“VR,” scenario typically involves presentation of digital or virtualimage information without transparency to other actual real-world visualinput; an augmented reality, or “AR,” scenario typically involvespresentation of digital or virtual image information as an augmentationto visualization of the actual world around the user.

The wearable device may include augmented and/or virtual realityglasses. The image can be displayed using image frames or raster scannedimages. In a scanning image display system, each of the light beamsdefines the pixels of the image. By scanning the mirrors in twoorthogonal axes, a two-dimensional field of view can be created. Theimages can be projected onto the spectacle lens, which can includewaveguide-based eyepieces and other optical elements, such as opticalfibers. The image display systems can be mounted on each of the left andright sides of the glasses frames.

SUMMARY OF THE INVENTION

In a scanning image display system employing a scanning projector and awaveguide eyepiece, the image light beam undergoes total internalreflection (TIR) inside the waveguide eyepiece. At every reflectionpoint where the image light beam reaches the output coupling element, abeamlet is out-coupled from the waveguide. If the density of theseoutput light beamlets is low, i.e., the resolution of the outputwavefront is low, then the image quality is poor. For example, the imageat the depth plane when viewed through the viewing box suffers from a“screen door” artifact, or wavefront sparsity artifact. To the user,this looks like viewing an image through a screen door.

Some embodiments of the present invention provide a method and systemfor increasing dynamic digitized wavefront resolution, i.e., the densityof output beamlets, by offsetting and replicating a collimated sourcelight beam. The source can be copied, or replicated, or cloned to formmultiple beamlets, and the beamlets are offset or displaced laterallysuch that there are effectively multiple collimated beam sources. Thismethod provides a way of increasing beamlet density independent of thesubstrate thickness. It also accentuates the focal/accommodation cuesthe eyepiece delivers to the eye.

Alternatively, certain embodiments of the invention provide a method andsystem for increasing output beamlet density through wavelengthdiversity. A collimated incoming source light beam can include multipleinput beams with different wavelengths in the vicinity of the nominalwavelength of a particular color. The incoming source light beam can bein-coupled into the eyepiece designed for the nominal wavelength. Theinput beamlets with multiple wavelengths diffract slightly differentlywhen in-coupling into the waveguide and thus take different paths whenthey undergo total internal reflection in the waveguide, and couple outat distinct positions to produce multiple offset output beamlets.

According to some embodiments of the invention, an image display systemincludes a waveguide, and an optical device configured for receiving anincoming light beam and providing a plurality of input beamlets to thewaveguide. Each input beamlet is derived from a portion of the incominglight beam, and the input beamlets are offset spatially from each other.The waveguide is configured for receiving the plurality of inputbeamlets using an input coupling element, propagating the plurality ofinput beamlets by total internal reflection (TIR), and outputtingmultiple groups of output beamlets using an output coupling element.Each group of output beamlets includes a portion of each of theplurality of input beamlets propagating in the waveguide by totalinternal reflection.

In some embodiments of the above image display system, the opticaldevice includes a first surface and a second surface disposed inparallel and adjacent to each other. The first surface is partiallyreflective, and the second surface is substantially totally reflective.In some embodiments, the partially reflective first surface isconfigured to receive a light beam, and to reflect a first portion ofthe received light beam and to allow a second portion of the receivedlight beam to pass through. The second surface is configured to reflecteach light beam it receives from the first surface back to the firstsurface. For each light beam directed to the first surface from thesecond surface, the partially reflective first surface is configured toallow a portion to pass through toward the waveguide to form a newbeamlet, and to reflect a remaining portion to the second surface.

In some embodiments of the above image display system, the opticaldevice further includes a third surface and a fourth surface disposed inparallel and adjacent to each other, the third surface being partiallyreflective and the fourth surface being substantially totallyreflective. The first and second surfaces are configured to receive theincoming light beam and provide a first plurality of beamlets. The thirdand fourth surfaces are configured to receive each of the firstplurality of beamlets and provide multiple beamlets.

In some embodiments of the above image display system, the first surfaceand the second surface are configured to form an incoupling angle with atop surface of the waveguide such that no input grating in the waveguideis needed.

According to some embodiments of the invention, a method for displayingan image includes providing a waveguide, receiving an incoming lightbeam and providing a plurality of input beamlets to the waveguide, theinput beamlets being offset spatially. The method also includesreceiving the plurality of input beamlets in the waveguide, andpropagating the plurality of input beamlets by total internal reflection(TIR) along different paths in the waveguide. The method furtherincludes outputting multiple groups of output beamlets using an outputcoupling element. Each group of output beamlets includes a portion ofeach of the plurality of input light beams propagating in the waveguideby total internal reflection.

According to some embodiments of the invention, an alternative imagedisplay system includes a waveguide, and a light source for providing acollimated incoming light beam that includes a plurality of input lightbeams having different wavelengths. The waveguide is configured forin-coupling the plurality of input light beams into the waveguide usinga wavelength-sensitive input coupling element, and propagating theplurality of input beams by total internal reflection (TIR), with eachinput light beam propagating along a different path in a differentdirection. The system is also configured for outputting multiple groupsof output light beamlets using an output coupling element. Each group ofoutput light beamlets includes a portion of each of the plurality ofinput light beams propagating in the waveguide by total internalreflection.

In an embodiment of the above system, wherein the waveguide isconfigured for a nominal wavelength of a color, and the plurality ofinput light beams have wavelengths in the vicinity of the nominalwavelength.

According to some embodiments of the invention, a method for displayingan image includes providing a waveguide, and providing a collimatedincoming light beam. The collimated incoming light beam includes aplurality of input light beams having different wavelengths. The methodalso includes in-coupling the plurality of input light beams into awaveguide using a wavelength-sensitive input coupling element, andpropagating the plurality of input beamlets by total internal reflection(TIR). Each beamlet is configured to propagate along a different path.The method also includes outputting multiple groups of output beamletsusing an output coupling element. Each group of output beamlets includesa portion of each of the plurality of input light beams propagating inthe waveguide by total internal reflection.

Additional features, benefits, and embodiments are described below inthe detailed description, figures, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram illustrating a perspective viewof an exemplary wearable display device according to some embodiments ofthe present invention;

FIG. 2 is a simplified schematic diagram illustrating scanning displaysystem according to some embodiments of the present invention;

FIG. 3 is a simplified schematic diagram illustrating an image displaysystem according to some embodiments of the present invention;

FIGS. 4A-4D illustrate output beamlets wavefronts from an image displaysystem according to some embodiments of the present invention;

FIGS. 5A-5D are simplified schematic diagrams illustrating image displaysystems according to some embodiments of the present invention;

FIGS. 6A and 6B are images illustrating the reduction of the screen dooreffect by the optical device described above according to someembodiments of the present invention;

FIG. 6C is a simplified drawing illustrating an experimental system forverifying the function of the display system according to an embodimentof the present invention;

FIG. 7A is a simplified schematic diagram illustrating another opticaldevice for producing multiple output beamlets according to someembodiments of the present invention;

FIG. 7B is a simplified schematic diagram illustrating another opticaldevice for producing multiple output beamlets according to someembodiments of the present invention

FIG. 8 is a simplified flowchart illustrating a method for displaying animage with reduced wavefront sparsity effect or screen door effectaccording to an embodiment of the present invention;

FIG. 9 is a simplified schematic diagram illustrating an image displaysystem according to an alternative embodiment of the present invention;

FIGS. 10A and 10B are images illustrating the reduction of the screendoor effect by the optical device described above according to someembodiments of the present invention; and

FIG. 11 is a simplified flowchart illustrating a method for displayingan image with reduced wavefront sparsity effect or screen door effectaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Representative applications of methods and apparatus according to thepresent application are described in this section. These examples arebeing provided solely to add context and aid in the understanding of thedescribed embodiments. It will thus be apparent to one skilled in theart that the described embodiments may be practiced without some or allof these specific details. In other instances, well known process stepshave not been described in detail in order to avoid unnecessarilyobscuring the described embodiments. Other applications are possible,such that the following examples should not be taken as limiting.

FIG. 1 is a simplified schematic diagram illustrating a perspective viewof an exemplary wearable display device 100 according to someembodiments of the present invention. Wearable display device 100includes main displays 110. In some embodiments, wearable display device100 also includes projector assemblies 120, which are integrated intotemple arms 130. Projector assemblies 120 can include projectors thatshine light through diffractive optics that is then reflected into theeyes of a user through main displays 110.

FIG. 2 is a simplified schematic diagram illustrating a scanning displaysystem according to some embodiments of the present invention. In thisexample, scanning display system 200 can be part of an eyepiece, e. g.,a waveguide based eyepiece, in a wearable device, such as wearabledevice 100 in FIG. 1 . As shown in FIG. 2 , scanning display system 200includes a scanning projector 210 configured to emit light beams, e.g.,beam 240, across a surface 250 to project an image. In some embodiments,scanning projector 210 can be part of projector assemblies 120 inwearable display device 100 in FIG. 1 .

FIG. 3 is a simplified schematic diagram illustrating an image displaysystem according to some embodiments of the present invention. FIG. 3shows an image display system 300 that includes a side view of aneyepiece with a waveguide 310. A single collimated light beam 320 isdirected toward waveguide 310. Light beam 320 can be provided by ascanning projector, such as a fiber scanner. Alternatively, light beam320 can also be provided by a collimated light reflected off a scanningmirror. The image display system 300 further includes an input couplingelement 312 configured to admit incident light for propagation by totalinternal reflection (TIR), as well as an output coupling element 314configured to expand and outcouple light propagating along waveguide 310by TIR toward a user's eye 390. Light beam 320 enters waveguide 310 atinput coupling element 312 of waveguide 310 and undergoes total internalreflection (TIR) as shown by the arrows 322 inside waveguide 310. Anarray of beamlets 330 are out-coupled at output coupling element 314 inthe exit pupil of waveguide 310. The array of output beamlets 330 formsa wavefront. In some embodiments, the image display system can alsoinclude optical elements 380, such as an eye lens, that directs theimage to the user's eye 390. From the user's perspective, the array ofoutput beamlets form a two-dimensional wavefront, as described furtherin FIGS. 4A-4D. The density of output beamlets is determined by thebounce spacing b, which in-turn is determined by substrate thickness t.

The input and output coupling elements 312 and 314 may be diffractiveoptical elements (“DOEs”), e.g., linear gratings, embedded within orimprinted upon waveguide 310. In some examples, the image display system300 may further comprise an orthogonal pupil expander (“OPE”) element(not shown) in addition to the output coupling element 314 to expand thelight in both the X and Y directions. The output coupling element 314may be slanted in the Z plane (i.e., normal to the X and Y directions)such that beamlets that are propagating through waveguide 300 will bedeflected by 90° in the Z plane and toward the user's eye 390. Theoutput coupling element 314 is also partially transparent and partiallyreflective along the light path (the Y axis), so that beamlets partiallypass through the output coupling element 314 to form multiple beamletsspaced distance b apart from one another. More details about inputcoupling elements, output coupling elements, OPEs, and other DOEs aredescribed in U.S. Utility patent application Ser. No. 14/555,585 andU.S. Utility patent application Ser. No. 14/726,424, the contents ofwhich are incorporated herein by reference in their entirety.

FIGS. 4A-4D illustrate output beamlets wavefronts from an image displaysystem according to some embodiments of the present invention. FIG. 4Aillustrates a waveguide of low density output beamlets forming a sparsewavefront, also referred to as having a low resolution wavefront. Inthis case, the image at the depth plane when viewed through the viewingbox suffers from a “screen door” artifact, also known as a wavefrontsparsity artifact (which looks like viewing image through a screendoor). FIG. 4B is an image illustrating the screen door effect caused bywavefront sparsity. This problem is especially severe for narrowin-coupled beams that are highly monochromatic (e.g., from a fiberscanner). For comparison, FIG. 4C illustrates a waveguide of a higherdensity of output beamlets, and FIG. 4D illustrates a waveguide of amuch higher density of output beamlets. In a real world, the images canhave essentially infinite resolution, which can offer strong depth cues.

As described above, a sparse wavefront causes undesired image artifactsin which the image appears as if it is being viewed through a screendoor. A straightforward way to increase the beam density is to reducethe substrate thickness. As the rays bounce back and forth between thetwo surfaces of the waveguide for a given angle, the pitch or bouncespacing gets smaller as the two parallel surfaces are closer together,i.e., as thickness of waveguide decreases. However, below a certainthickness, further reducing thickness of substrate becomes prohibitivelychallenging and introduces other image quality and manufacturing issues.Embodiments of the invention provide techniques to increase wavefrontresolution, independent of substrate thickness, as described below.

According to some embodiments of the present invention, in order toincrease dynamic digitized wavefront resolution and mitigate screendoor/wavefront sparsity artifacts, the incoming light beam can becopied, replicated, or cloned and offset or displaced laterally suchthat now there are effectively multiple collimated beam sources. Thisprovides a way of increasing beamlet density independent of substratethickness. It also accentuates the focal and accommodation cues theeyepiece delivers to the eye of a user.

FIG. 5A is a simplified schematic diagram illustrating an image displaysystem according to some embodiments of the present invention. As shownin FIG. 5A, an image display system 500 includes a waveguide 510 and anoptical device 550 configured for receiving an incoming light beam 520and providing a plurality of input beamlets 521 to the waveguide 510.Each image beamlet 521 is derived from a portion of incoming light beam520. As shown in FIG. 5A, four source beamlets 521 are offset spatially,so that they are directed to different locations on the input couplingelement 512 optically coupled to waveguide 510. Although only foursource beamlets are illustrated in FIG. 5A, embodiments of the presentinvention are not limited to this particular number of source beamlets.In other embodiments, a reduced number of source beamlets are utilized,while in yet other embodiments, an increased number of source beamletsare utilized. As discussed in additional detail with respect to FIG. 5C,in some embodiments, the number of source beamlets is a function of thespatial parameters associated with optical device 550, additionaldetails of which are described more fully with respect to FIGS. 5B-5D.

In FIG. 5A, waveguide 510 is configured for receiving the plurality ofinput beamlets 521 at an input coupling element 512, e. g., an inputcoupling grating. In the illustrated embodiment, the source beamlets areincident on the input coupling element 512 at normal incidence. However,this is not required by the present invention and operation at otherangles of incidence are included with the scope of the presentinvention. Source beamlets 521 diffract upon passing through inputcoupling element 512 at a non-normal angle of propagation insidewaveguide 510.

After passing through and diffracting from input coupling element 512,input beamlets 521 propagate along waveguide 510 by total internalreflection (TIR), with the reflected beams shown as 522 in FIG. 5Amaking multiple passes as they propagate from the end of waveguide 510adjacent the input coupling element 512 toward the right end of outputcoupling element 514 optically coupled to the lower surface of waveguide510. The waveguide 510 can be characterized by a longitudinal axisaligned with the direction of propagation of light along the waveguide.In FIG. 5A, the longitudinal axis 505 is aligned with the top and bottomsurfaces of the waveguide and is parallel to the direction ofpropagation of incoming light beam 520.

In the waveguide, the plurality of input beamlets propagates along thewaveguide by total internal reflection (TIR), in which a beamlet istotally reflected back internally, when it reaches a surface of thewaveguide. The phenomena occurs if the angle of incident of the beamletis greater than a critical angle of the waveguide. In FIG. 5B, each ofthe plurality of beamlets 521 is represented by a different solid ordashed line pattern, and each of the plurality of input beamletstraverses a different path in the waveguide.

Waveguide 510 also is configured to output multiple groups of outputbeamlets 530 using an output coupling element 514, e.g., an outputcoupling grating. Output coupling element 514 is coupled to waveguide510 at a surface of the waveguide. Output coupling element 514 causesthe beamlets 522 in the waveguide to be partially refracted at thesurface to exit the waveguide and partially reflected back into thewaveguide. In FIG. 5A, when the beamlets inside the waveguide firstreach output coupling element 514, a portion of each beamlet isrefracted and exits the waveguide to form a first group of outputbeamlets 531. The remaining portions of the beamlets continue topropagate by total internal reflection and form subsequent groups ofoutput beamlets 532 to 534 when they exit the waveguide at differentpositions along the longitudinal axis. Therefore, each group of outputbeamlets includes a portion of each of the plurality of input lightbeamlets propagating in the waveguide by total internal reflection. Forexample, four groups of output beamlets, 531, 532, 533, and 534, areshown in FIG. 5A. Each group of output beamlets includes a portion ofeach of the plurality of input beamlets 521 propagating in the waveguideby total internal reflection.

FIG. 5B is a simplified schematic diagram illustrating an image displaysystem according to some embodiments of the present invention. As shownin FIG. 5B, an image display system 560 is similar to image displaysystem 500 in FIG. 5A, but with optical device 550 replaced with aspecific implementation. In this example, optical device 550 includes afirst surface 552 and a second surface 554 disposed in parallel,adjacent to each other, and at an oblique angle, e.g., at a 45° angle,to one or more surfaces of waveguide 510, input coupling element 512,and/or output coupling element 514. The first surface 552 is partiallyreflective, and the second surface 554 is substantially totallyreflective. The operation of optical device 550 is explained furtherwith reference to a magnified view of optical device 550 as shown inFIG. 5C.

FIG. 5C is a simplified schematic diagram illustrating the opticaldevice 550 in the image display system of FIG. 5B according to someembodiments of the present invention. In FIG. 5C, the partiallyreflective first surface 552 and the reflective second surface 554 areprovided by two prisms 562 and 561, respectively, which are righttriangular and comparable in size. As such, in some examples the opticaldevice 550 may be cuboidal or quasi-cuboidal in shape. An incoming lightbeam 520 enters optical device 550 and provides a plurality of imagebeamlets 521 that are offset spatially. Each image beamlet 521 isderived from a portion of incoming light beam 520. In this example,incoming light beam 520 is reflected by the reflective second surface554 toward the partially reflective surface 552 at point A1. Thepartially reflective first surface 552 reflects a first portion of theincoming light beam to the second surface, and allows a second portionof the incoming light beam to transmit through the prism to form a firstbeamlet B1. Similarly, the light beam reflected from the second surface554 to reach point A2 of the first surface 552 is partially reflectedtoward the second surface 554, and partially passes through prism 562 toform a second beamlet B2. In a similar manner, part of the light beamreaching point A3 forms a third beamlet B3, and part of the light beamreaching point A4 forms a fourth beamlet B4. For illustration purposes,in the example of FIG. 5C, the reflectivity of the partially reflectivesurface 552 is presumed to be 50%. As a result, the intensities of lightbeamlets B1, B2, B3, and B4 are ½, ¼, ⅛, and 1/16, respectively, of theintensity of incoming light beam 520, as shown in FIG. 5C.

The intensity distribution in FIG. 5C is derived based on the partiallyreflective surface 552 having a reflectivity of 50%. In someembodiments, the reflectivity can be varied to lead to different beamletintensity distributions. In some embodiments, the reflectivity along thepartially reflective surface 552 can be varied to achieve a desiredintensity distribution.

The partially reflective first surface 552 can include a partiallyreflective coating, such as one composed of a metal, e.g., gold,aluminum, silver, nickel-chromium, chromium, etc., a dielectric, e.g.,oxides, fluorides, sulfides, etc., a semiconductor, e.g., silicon,germanium, etc., and/or a glue or adhesive with reflective properties,which can be applied to prism 562 by way of any suitable process, e.g.,physical vapor deposition (“PVD”), ion-assisted deposition (“IAD”), ionbeam sputtering (“IBS”), etc. The ratio of reflection to transmission ofsuch a coating may be selected or determined based at least in part uponthe thickness of the coating, or the coating may have a plurality ofsmall perforations to control the ratio of reflection to transmission.It follows that the output coupling element 514 may include a partiallyreflective coating composed of one or more of the abovementionedmaterials. The reflective second surface 554 can include a reflectivecoating, which may also be composed of one or more of the abovementionedmaterials, but thick enough so as to sufficiently render the secondsurface 554 completely or almost completely reflective. In someembodiments, surfaces 552 and 554 of prisms 562 and 561, respectively,can be directly or indirectly bonded together with glue or adhesive,such as a glue or adhesive with reflective properties as describedabove.

In some embodiments, the number of input beamlets can be changed byvarying the spacing between the partially reflective surface 552 and thereflective surface 554. For example, reducing the spacing between thetwo surfaces can lead to an increased number of reflections between thetwo surfaces, generating more input beamlets. In FIGS. 5A-5C, thepartially reflected first surface 552 and the reflective second surface554 are represented by planar surfaces. In other embodiments, thepartially reflected first surface and the reflective second surface canhave different shapes, for example, parabolic, spherical, or othershapes.

In FIG. 5C, incoming light beam 520 reaches the second surface 554 andis reflected toward the first surface 552. Alternatively, if theincoming light beam 520 enters prism 562 before reaching totallyreflective surface 554, as shown in FIG. 5D, then the partiallyreflective first surface 552 is configured to reflect a first portion ofthe incoming light beam toward the waveguide to form a first beamlet,and to allow a second portion of the incoming light beam to transmit tothe second surface. Subsequent beamlets are formed in the mannerdescribed above in connection with FIG. 5C.

FIGS. 6A and 6B are images illustrating the reduction of the screen dooreffect by the optical device described above according to someembodiments of the present invention. FIG. 6A is the same as FIG. 4B andillustrates the screen door effect caused by wavefront sparsity. FIG. 6Bis an image illustrating the reduction of screen door effect byproviding multiple beamlets from a single incoming light beam. Thebeamlet intensity is plotted below the images. In FIG. 6B, the beamletintensity is based on FIG. 5C where the partial reflectivity mirror isassumed to have a reflectivity of 0.5. In embodiments of the invention,more improvements can be achieved by varying beamlet intensitydistribution and increasing the number of beamlets derived from theincoming light beam. These improvements can be obtained by optimizingthe shape and reflectivity of the reflective and partially reflectivesurfaces.

FIG. 6C is a simplified drawing illustrating an experimental system forverifying the function of the display system described above. FIG. 6Cshows a single collimated light source 610 from a fiber scanner and anoptical device 620. Collimated light source 610 provides a plurality ofcollimated light beams 611. Optical device 620 includes two prisms 621and 622, respectively, providing a partially reflective mirror 623 and a100% reflective mirror 624. A certain transverse distance is maintainedbetween the two parallel planes 623 and 624 from the two mirrors.Optical device 620 receives each of the plurality of collimated lightbeams 611 and produces multiple beamlets 630 that are projected onto animage sensor 640. FIGS. 6A and 6B are examples of images that can beprovided by image sensor 640.

FIG. 7A is a simplified schematic diagram illustrating an optical devicefor producing multiple output beamlets according to some embodiments ofthe present invention. As shown in FIG. 7A, optical device 700 can beused as optical device 550 in the image display system of FIGS. 5A and5B. Optical device 700 can include multiple “optical devices,” each ofwhich can be similar to optical device 550 in optical device 700. Themultiple optical devices can be positioned in a cascading arrangement toprovide additional beam cloning. As shown in FIG. 7A, prisms 701 and 702form a first optical device 721, and prisms 703 and 704 form a secondoptical device 722. Both first optical device 721 and second opticaldevice 722 are similar to optical device 550, but are oriented indifferent directions.

In FIG. 7A, optical device 721 includes prism 701 with a partiallyreflected surface 711 and prism 702 with a reflective surface 712.Similarly, optical device 722 includes prism 703 with a partiallyreflected surface 713 and prism 704 with a reflective surface 714. Insome examples the optical devices can be oriented differently from oneanother such that a 2D array of cloned beamlets can be in-coupled into awaveguide. For example, an incoming light beam 710 enters optical device721, which provides a plurality of image beamlets (not shown) that areoffset spatially and traverse along a longitudinal direction along thewaveguide. Each of the beamlets from optical device 721 entering opticaldevice 722 can provide a plurality of beamlets that forms a portion of aplurality of input beamlets 730 that are directed to the waveguide. As aresult, a 2D array of cloned beamlets can be in-coupled into awaveguide.

As an example, optical device 721 can be characterized by a cloningfactor or multiplicity factor of m, i.e., a single input beam canproduce m output beamlets, and optical device 722 can be characterizedby a cloning factor or multiplicity factor of n. Then, the cascadedoptical device 700 can have a cloning factor or multiplicity factor ofm×n. As shown in FIGS. 5A and 5B, each input beamlet entering thewaveguide produces multiple output beamlets emitting from the waveguide.The wavefront density can be greatly increased. In some embodiments,more than two optical devices can be cascaded to further increase thewavefront density.

Although described primarily within the context of triangular prisms,particularly right triangular or “Porro” prisms, it is to be understoodthat one or more of the prisms described herein may take on othergeometries. For instance, a single Porro-Abbe prism can be implementedwith four triangular prisms with totally reflective surfaces that areeach arranged parallel to a respective one of the Porro-Abbe prism'sfour hypotenuse sides, so as to provide the beam cloning functionalityof four optical devices in a cascading configuration. Other geometriescan include any of a variety of other polyhedral geometries, such as“Amici” or Amici roof prism geometries, parallelogram prism geometries,trapezoidal prism geometries, pyramidal or semi-pyramidal prismgeometries, e.g., tetrahedral prism geometries, diagonal slices ofcuboidal or triangular prism geometries, and the like.

FIG. 7B is a simplified schematic diagram illustrating another opticaldevice for producing multiple output beamlets according to someembodiments of the present invention. In these embodiments, the bottomprism of the optical device can provide incoupling functionality. Asshown in FIG. 7B, an optical system 750 includes an optical device 760for producing multiple beamlets from an input light beam and a waveguide780 for receiving the multiple beamlets. Optical device 760 includes afirst prism 761 with a partially reflected surface 771 and a secondprism 772 with a reflective surface 772. The prisms can take on ageometry other than a 45°-45°-90° triangular prism, such that the prismsreflect an incoming light 790 into the waveguide 780 at an desiredoblique incoupling angle, e.g., different from a 90° angle. As shown inFIG. 7B, surfaces 771 and 772 of the prisms form an angle α of less than45° with at top surface of waveguide 780, which can lead the beamlets toenter the waveguide at a slanted incident angle. As such, in some ofthese embodiments, the incoupling grating may not be necessary. Othergeometries/configurations for providing incoupling functionality by wayof the optical device 760 and/or waveguide 780 can also be used. Forexample, surfaces 771 and 772 of the prisms form an angle α of greaterthan 45° with at top surface of waveguide 780. In another example, aninput coupling element, e.g., grating, can be provided on the surface ofone or both legs, e.g., the non-hypotenuse sides, of the first prism 761(which is right triangular in shape). In yet another example, the firstprism 761 can take on a geometry other than a triangular prism, such asthat of a parallelogram. In such an example, the first prism 761 may bearranged so as to incouple light into a side surface of waveguide 780,as opposed to a top surface of waveguide 780. In some examples, thefirst prism 761 may be effectively integral to waveguide 780.

FIG. 8 is a simplified flowchart illustrating a method for displaying animage with reduced wavefront sparsity effect or screen door effectaccording to an embodiment of the present invention. The method 800 fordisplaying an image includes providing a waveguide and an optical device(810) and receiving an incoming light beam (820). An examples opticaldevice is shown in FIGS. 5B and 5C, in which optical device 550 includesa first surface 552 and a second surface 554 disposed in parallel andadjacent to each other. In this example, the first surface 552 ispartially reflective, and the second surface 554 is substantiallytotally reflective. In some embodiments, the incoming light beam can beprovided using a fiber scanner.

The method also includes directing the incoming light beam to impinge ona portion of the optical device (830) and generating a plurality ofinput beamlets using the optical device (840). Referring to FIG. 5C,incoming light beam 520 is directed to the second surface 554 of opticaldevice 550 and is reflected toward the first surface 552, where thelight is partially reflected back toward the second surface 554. Theother portion of the light is transmitted through prism 562 and exitsprism 562 as a first input beamlet B1. Subsequent input beamlets B2, B3,and B4 are generated in a similar manner. The plurality of inputbeamlets are offset spatially with respect to each other. As shown inFIG. 5C, input beamlets B1 through B4 exit prism 562 at increasingdistances from the left side of prism 562 such that each of the inputbeamlets is positioned to impinge on the input coupling element at adifferent longitudinal positions arrayed along the longitudinal axis.

The method also includes coupling the plurality of input beamlets intothe waveguide (850) and propagating the plurality of input beamletspropagate along the waveguide by total internal reflection (860).Referring to FIG. 5B, input beamlets 521 are coupled into waveguide 510through input coupling element 512. In the waveguide, the plurality ofinput beamlets propagates along the waveguide by total internalreflection (TIR). In TIR, a beamlet is totally reflected backinternally, when it reaches a surface of the waveguide. In FIG. 5B, eachof the plurality of beamlets 521 is represented by a different solid ordashed line pattern, collectively labeled 522, and each of the pluralityof input beamlets traverses a different path in the waveguide.

Method 800 further includes providing an output coupling elementoptically coupled to the waveguide (870) and outputting multiple groupsof output beamlets using the output coupling element (880). As shown inFIG. 5B, output coupling element 514 is coupled to waveguide 510 at aninterface between the waveguide and ambient air. Output coupling element514 causes the beamlets 522 in the waveguide to be partially refractedat the boundary surface to exit the waveguide, and partially reflectedback into the waveguide. In FIG. 5B, when the beamlets inside thewaveguide first reach exit coupling element 514, a portion of eachbeamlet is refracted and exits the waveguide to form a first group ofoutput beamlets 531. The remaining portions of the beamlets continue topropagate by total internal reflection and form subsequent groups ofoutput beamlets 532 to 534 at different positions along the longitudinalaxis. Therefore, each group of output beamlets includes a portion ofeach of the plurality of input light beamlets propagating in thewaveguide by total internal reflection.

Examples of an image display system that implements method 800 aredescribed above in connection with FIGS. 3-7 . In some embodiments, theabove method includes providing an optical device for receiving anincoming light beam and providing a plurality of input beamlets that areoffset spatially. The plurality of input beamlets can be directed to awaveguide to generate increased number of output beamlets, which canincrease the wavefront density and reduce the screen door effect. Insome embodiments, the method can also include focusing the multiplegroups of output light beamlets using an eye lens.

It should be appreciated that the specific steps illustrated in FIG. 8provide a particular method for displaying an image with reducedwavefront sparsity effect or screen door effect according to anembodiment of the present invention. Other sequences of steps may alsobe performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 8 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

FIG. 9 is a simplified schematic diagram illustrating an image displaysystem according to an alternative embodiment of the present invention.As shown in FIG. 9 , an image display system 900 includes a waveguide910 configured for receiving an incoming light beam and a light source905 for providing a collimated incoming light beam 920. Collimatedincoming light beam 920 includes a plurality of input light beams havingdifferent wavelengths. As an example, in FIG. 9 , collimated incominglight beam 920 includes a first light beam 921 having a first wavelengthand a second light beam 922 having a second wavelength.

As shown in FIG. 9 , two input light beams 921 and 922 with slightlydifferent wavelengths are included in collimated incoming light beam920, so that they are diffracted slightly differently in the inputcoupling element 912 and enter waveguide 910 at slightly differentangles of incidence. Although only two input light beams are illustratedin FIG. 9 , embodiments of the present invention are not limited to thisparticular number of input light beams. In other embodiments, anincreased number of input light beams are utilized. As can be seen inFIG. 9 , increasing the number of input light beams can increase thenumber of output beamlets and further increase wavefront density of theoutput image.

In some embodiments, the different wavelengths are selected from a rangeof wavelengths centered around a nominal wavelength for which awaveguide eyepiece is designed. In an embodiment, for a red imagesignal, lasers of wavelengths 630 nm, 635 nm, and 640 nm can bein-coupled into a waveguide eyepiece nominally designed for 635 nm. Inanother embodiment, a single collimated incoming light beam can includecomponent light beams having wavelengths of 635 nm and 642 nm. In someembodiments, the plurality of input light beams can have wavelengthsthat span a spectrum width of about 5 nm, 10 nm, or 20 nm in thevicinity of a nominal wavelength. In some embodiments, the plurality ofinput light beams can have wavelengths that span a spectrum width ofabout 30 nm, or 50 nm in the vicinity of a nominal wavelength. In theseembodiments, the plurality of input light beams can be used to generateincreased number of output beamlets that can increase the wavefrontdensity of the image for the nominal wavelength. A higher number ofinput light beams can be used to generate a higher number of outputbeamlets.

Waveguide 910 is configured for in-coupling collimated incoming lightbeam 920 into waveguide 910 using a wavelength-sensitive input couplingelement 912, e. g., an input coupling grating. The wavelength-sensitiveinput coupling element can be a diffraction grating whose diffractionproperties depend on the wavelength of the incoming light beam.Wavelength-sensitive input coupling element 912 causes first light beam921 and second light beam 922 to diffract at different angles as theyenter waveguide 910. In the illustrated embodiment, the collimatedincoming light beam is incident on the input coupling element 912 atnormal incidence. However, this is not required by the present inventionand operation at other angles of incidence are included with the scopeof the present invention. Collimated incoming light beam is diffractedupon passing through input coupling element 912 at a non-normal angle ofpropagation inside waveguide 910.

The plurality of input light beams in collimated incoming light beam 920are configured to propagate in waveguide 910 by total internalreflection (TIR) along different paths in different directions inwaveguide 910. As shown in FIG. 9 , first light beam 921 and secondlight beam 922 enter waveguide 910 at different angles. As a result,light beams 921 and 922 have different incident angles as they reach thesurface of waveguide 910. Therefore, each of the input light beamstraverses a different path in the waveguide, as shown in FIG. 9 .

After passing through and diffracting from input coupling element 912,input light beams 921 and 922 propagate along waveguide 910 by totalinternal reflection (TIR), making multiple passes as they propagate fromthe end of waveguide 510 adjacent the input coupling element 912 towardthe right end of output coupling element 514 optically coupled to thelower surface of waveguide 910. The waveguide 910 can be characterizedby a longitudinal axis aligned with the direction of propagation oflight along the waveguide. In FIG. 9 , the longitudinal axis 905 isaligned with the top and bottom surfaces of the waveguide and isperpendicular to the direction of propagation of incoming light beam920.

Waveguide 910 is further configured to output multiple groups of outputlight beams 930 using an output coupling element 914. As shown in FIG. 9, output coupling element 914 is coupled to waveguide 910 at a lowersurface of the waveguide, and output coupling element 914 extendslongitudinally along waveguide 910. Since each of the input light beamstraverses a different path in the waveguide, they reach output couplingelement 914 at different locations, where a portion of each beamlet isrefracted and exits the waveguide to form an output beamlet and theremaining portion continues to propagate in the waveguide by TIR. FIG. 9shows multiple groups of output light beams 930, including groups 950,960, 970, and 980. Each group of output light beams includes a portionof each of the plurality of input light beams propagating in thewaveguide by total internal reflection. For example, output beamletgroup 950 includes a first beamlet 951 from part of incoming light beam921 and a second beamlet 952 from part of incoming light beam 922.Similarly, output beamlet group 960 includes a first beamlet 961 frompart of incoming light beam 921 and a second beamlet 962 from part ofincoming light beam 922. Output beamlet group 970 includes a firstbeamlet 971 from part of incoming light beam 921 and a second beamlet972 from part of incoming light beam 922. Output beamlet group 980includes a first beamlet 981 from part of incoming light beam 921 and asecond beamlet 982 from part of incoming light beam 922.

It can be seen that image display system 900 includes multiple inputlight beams having different wavelengths in the incoming collimatedlight beam 920 and a wavelength-sensitive input coupling element 912. Byusing a wavelength-sensitive input coupling element, the number ofoutput beamlets can be increased. As a result, the wavefront sparsity orscreen door effect can be reduced. The wavelength-sensitive inputcoupling element can be a diffraction grating whose diffractionproperties depend on the wavelength of the incoming light beam.

FIGS. 10A and 10B are images illustrating the reduction of the screendoor effect by the optical device described above according to someembodiments of the present invention. In order to verify the function ofthe image display system described above, an experiment was carried out,in which an incoming light beam is provided by a combiner that receivedlight of wavelengths 635 nm and 642 nm. The images from the waveguideeyepiece are viewed through a pinhole. FIG. 10A illustrates the screeneffect caused by wavefront sparsity. The image appears as a sparselysampled version of the original image. FIG. 10B is an image illustratingthe reduction of the screen door effect by providing a single collimatedincoming light beam that includes component light beams havingwavelengths 635 nm and 642 nm. In this example, for two lasers withwavelengths 7 nm apart, there is a noticeable shift in the anglesescaping from the pinhole, seen as additional spots in FIG. 10B.

As described above, the wavefront resolution is increased because, for asingle angle beam, there is a set of beamlets that forms the originalwavefront, but, with the addition of a second wavelength, there is ashifted set of beamlets that effectively increases the overallresolution of the wavefront corresponding to that input angle. This willimprove the “screen door” or more correctly “wavefront sparsity”problem. More improvements can be achieved by increasing the number ofbeamlets with different wavelengths in the incoming collimated lightbeam. For example lasers of 630 nm, 635 nm and 640 nm can be in-coupledinto a waveguide eyepiece nominally designed for 635 nm. In embodimentsof the invention, light sources, such as Lasers, with a spectrum widthof about 20 nm will significantly improve image quality. For eyepieceswith no lensing function, this provides a way of increasing beamletdensity independent of substrate thickness. For eyepieces with a lensingfunction, the focal plane for each wavelength is slightly different andcould increase the depth of focus of the eyepiece.

FIG. 11 is a simplified flowchart illustrating a method for displayingan image with reduced wavefront sparsity effect or screen door effectaccording to an embodiment of the present invention. As shown in FIG. 11, the method 1100 for displaying an image includes providing a waveguidehaving a wavelength-sensitive input coupling element (1110). Referringto FIG. 9 , a waveguide 910 has a wavelength-sensitive input couplingelement 912. The wavelength-sensitive input coupling element can be adiffraction grating whose diffraction properties depend on thewavelength of the incoming light beam.

The method also includes providing a collimated incoming light beam(1120). The collimated incoming light beam includes a plurality of inputlight beams having different wavelengths. As an example, in FIG. 9 ,collimated incoming light beam 920 includes a first light beam 921having a first wavelength and a second light beam 922 having a secondwavelength. In some embodiments, the different wavelengths are selectedfrom a range of wavelengths centered around nominal wavelength for whicha waveguide eyepiece is designed. In an embodiment, for a red imagesignal, lasers of wavelengths 630 nm, 635 nm and 640 nm can bein-coupled into a waveguide eyepiece nominally designed for 635 nm. Inanother embodiment, a single collimated incoming light beam thatincludes component light beams having wavelengths of 635 nm and 642 nm.In some embodiments, the plurality of input light beams can havewavelengths that span a spectrum width of about 20 nm. In theseembodiments, the plurality of input light beams can be used to generateincreased number of output beamlets that can increase the wavefrontdensity of the image for the nominal wavelength.

The method also includes in-coupling the plurality of input light beamsinto a waveguide using the wavelength-sensitive input coupling element(1130). Referring to FIG. 9 , the wavelength-sensitive input couplingelement 912 is configured to in-couple collimated incoming light beam920, which includes a first light beam 921 having a first wavelength anda second light beam 922 having a second wavelength. Wavelength-sensitiveinput coupling element 912 causes first light beam 921 and second lightbeam 922 to diffract at different angles as they enter waveguide 910.

Method 1100 also includes propagating the plurality of input light beamsin the waveguide by total internal reflection (1140). As shown in FIG. 9, first light beam 921 and second light beam 922 enters waveguide 910 atdifferent angles. As a result, light beams 921 and 922 have differentincident angles as they reach a surface of waveguide 910. Therefore,each of the input light beams traverses a different path in thewaveguide, as shown in FIG. 9 .

The method further includes providing an output coupling elementoptically coupled to the waveguide (1150) and outputting multiple groupsof output beamlets using the output coupling element (1160). As shown inFIG. 9 , output coupling element 914 is coupled to waveguide 910 at asurface of the waveguide and extends longitudinally along waveguide 910.Since each of the input light beams traverse different paths in thewaveguide, they reach output coupling element 914 at differentlocations, where a portion of each beamlet is refracted and exits thewaveguide to form an output beamlet and the remaining portion continuesto propagate in the waveguide by TIR. As shown in FIG. 9 , multiplegroups of output light beams 930 is emitted from output coupling element914. The multiple groups of output light beams 930 include groups 950,960, 970, and 980. Each group of output light beams includes a portionof each of the plurality of input light beams propagating in thewaveguide by total internal reflection. For example, beamlet group 950includes a first beamlet 951 from part of incoming light beam 921 and asecond beamlet 952 from part of incoming light beam 922. Similarly,beamlet group 960 includes a first beamlet 961 from part of incominglight beam 921 and a second beamlet 962 from part of incoming light beam922. Beamlet group 970 includes a first beamlet 971 from part ofincoming light beam 921 and a second beamlet 972 from part of incominglight beam 922. Beamlet group 980 includes a first beamlet 981 from partof incoming light beam 921 and a second beamlet 982 from part ofincoming light beam 922.

An example of an image display system that implements method 1100 isdescribed above in connection with FIGS. 9 and 10 . By using awavelength-sensitive input coupling element, the number of outputbeamlets can be increased. The method can also include focusing themultiple groups of output light beamlets using an eye lens. Further, theincoming light beam can be provided with a fiber scanner. In someembodiments, the waveguide is configured for a nominal wavelength of acolor, and the plurality of input light beams have wavelengths in thevicinity of the nominal wavelength.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the describedembodiments. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice the describedembodiments. Thus, the foregoing descriptions of specific embodimentsare presented for purposes of illustration and description. They are notintended to be exhaustive or to limit the described embodiments to theprecise forms disclosed. It will be apparent to one of ordinary skill inthe art that many modifications and variations are possible in view ofthe above teachings.

What is claimed is:
 1. An image display system comprising: a waveguide;an optical device configured for receiving an incoming light beam andproviding a plurality of input beamlets to the waveguide, wherein theoptical device comprises: a first prism and a second prism separated byan air gap, a first surface of the first prism and a second surface ofthe second prism being disposed in parallel and adjacent to each otheracross the air gap, the first surface being partially reflective and thesecond surface being substantially totally reflective, wherein the firstprism and the second prism are configured to receive the incoming lightbeam and to provide a first plurality of beamlets; wherein the firstsurface is configured for receiving the incoming light beam andreflecting a first portion of the incoming light beam and to allow asecond portion of the incoming light beam to pass through; the secondsurface is configured for reflecting each light beam from the firstsurface back to the first surface; and for each light beam directed tothe first surface from the second surface, the first surface allowing aportion to pass through, and reflecting a remaining portion to thesecond surface; each beamlet of the first plurality of beamlets beingderived from a portion of the incoming light beam, the first pluralityof beamlets being offset spatially; a third prism and a fourth prism, athird surface of the third prism and a fourth surface of the fourthprism disposed in parallel and adjacent to each other, the third surfacebeing partially reflective and the fourth surface being substantiallytotally reflective; wherein the third prism and the fourth prism areconfigured to receive the first plurality of beamlets and to provide theplurality of input beamlets, wherein the first plurality of beamletsenter the third prism perpendicular to a first side surface of the thirdprism; and wherein the first plurality of beamlets is offset spatiallyand traverses across a longitudinal direction of the waveguide, and theplurality of input beamlets is offset along a latitudinal direction ofthe waveguide, resulting in a 2D (two-dimensional) array of beamlets;wherein the waveguide is configured for: receiving the plurality ofinput beamlets; propagating the plurality of input beamlets by totalinternal reflection (TIR); providing an output coupling elementoptically coupled to the waveguide; and outputting multiple groups ofoutput beamlets using the output coupling element, each group of outputbeamlets including a portion of each of the plurality of input beamletspropagating in the waveguide by total internal reflection.
 2. The imagedisplay system of claim 1, wherein the plurality of input beamlets isdirected in parallel toward the waveguide.
 3. The image display systemof claim 1, wherein the multiple groups of output beamlets are spacedapart by a bounce spacing of the waveguide.
 4. The image display systemof claim 1, wherein the partially reflective first surface ischaracterized by a uniform reflectivity.
 5. The image display system ofclaim 1, wherein the waveguide comprises a top surface, a portion ofwhich is configured for receiving the plurality of input beamlets fromthe optical device.
 6. The image display system of claim 5, wherein thethird surface and the fourth surface are configured to form anincoupling angle with the top surface of the waveguide.
 7. The imagedisplay system of claim 1, wherein the waveguide is positioned in alateral plane, the third and fourth surfaces being disposed at anoblique angle to the lateral plane.
 8. The image display system of claim1, wherein the plurality of input beamlets exit the third prismperpendicular to a second side surface of the third prism.
 9. The imagedisplay system of claim 8, wherein the second side surface of the thirdprism is parallel to the waveguide.
 10. The image display system ofclaim 8, wherein the first side surface of the third prism isperpendicular to the second side surface of the third prism.